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Making secret messages and IR t-shirts

Here's a trick. Take a sheet of Congo Blue filter and overlay it with Primary Red. It looks black to your unaided eyes. Now wear your IR filter goggles and observe those "black" filter sheets under incandescent light (or take them outdoors into sunlight.) You'll find that the sheets of Congo Blue plus Primary Red now appear to be transparent!
They look a bit grey rather than totally clear, so you might want to try using a different colored filter instead, for example use a couple of layers of Roscolux #385 "Royal Blue." Unaided eyes think the red/blue filter stack is totally black, but your IR goggles let you SEE RIGHT THROUGH the filter sheets. Write a secret message on a piece of paper and cover it up with the "black" IR filter stack. Normal humans will see nothing but a shiney black square. But with your goggles you can see the secret message. Make IR-only signs. "No Cyborgs beyond this point." "Human infants taste terrible!" "Chlorine-breathing reptoids out of US Congress!" Any eyes that possess, ahem, enhanced longwave response will see your hidden message, but all of the "normals" will just see a featureless black square.

Here's another way to do the same thing. First use the goggles to look at different kinds of dark clothing. Find some cloth that looks light grey in the infrared, but looks dark black when you take the goggles off. It's easy to make a secret message with this cloth. Just write on that black cloth using black magic marker. Human eyes can't see the black-on-black. But if you wear the goggles on a sunny day, then the black writing will be clearly visible against the light cloth. (Most black magic markers have ink which is black in both the visible and the IR.) You can draw anything you want to on your black clothing. Only people with IR goggles (or IR cameras) can see it. [NOTE: I made some signs like this, and I found that I can still see the lettering by eye if large block letters are used. The large regions of "sharpie marker" ink is still visible on the black cloth. Run the black cloth through the wash to reduce this problem. On the other hand, thin writing is still invisible. It's only the big black blocks that can be seen by humans if they're paying attention.]

Test for better filters

Here's a trick that demonstrates that you're really dealing with IR. If you have a "swatch pack" of Lee color filters, find the Congo Blue #181 and the Peacock Blue #115. To normal human eyes, Congo blue appears almost opaque black, and Peacock blue looks transparent sky-blue. Now wear your IR filter goggles and look again. (You'll need sunlight or an incandescent bulb for illumination.) You'll find that the Congo Blue filter is no longer opaque! It now looks transparent... but now you can't see through the Peacock Blue. In the IR band, their roles are reversed. The Peacock blue filter is a black absorber under IR light, while the Congo Blue is transparent. Look at other filters in the Lee filter swatch-pack. You're really seeing the IR transmission of these filters, and as with Peacock Blue, the ones which look black in the near-IR are often very transparent in the visible.

Goth-ray Vision

Remember that I mentioned that certain clothing looks black for human eyes, but looks white in the IR? Certain dark blue dyes act this way. Some new blue-jeans look white in the infrared, while black work-pants appear black, but in the visible spectrum they both look very dark. Find yourself a black windbreaker which appears white in the infrared. Use carbon-based ink to put a nasty message or some disturbing artwork on the back, and only IR cameras (and IR filter goggles) will see it. Do "Goth Warchalking", where you write on bluish-black paper with black magic markers, and the resulting messages are only visible to these weirdos who go around wearing black-lensed mad scientist goggles.

Now I need to find some IR-absorbing spray paint and magic markers. I want to do the opposite to the above. I want some kind of paint which looks totally water-clear to human eyes, but looks totally black at 720nM infrared. Why? Because then I can put IR graffiti all over everything, and nobody can see it unless they're equipped with IR goggles. I'll draw "crop circles" on lawns and city streets that only IR cameras can see! Maybe get some huge nasty tattoos on my face which are invisible to mundane eyes. Hmmmm, I wonder if anyone is already doing this. If I keep a lookout while wearing goggles maybe I'll find secret messages on city sidewalks written by the MIBs. Search google on "infrared paint."

A view more Infra-reddly

Congo blue filers give your eyes a peak sensitivity of around 720nM. That's definitely into the IR band which starts at 700. If you want your vision to be much deeper into the IR, you can use a different Lee or Rosco filter, one with an even deeper IR cutoff. One such filter is Lee #120 "Deep Blue." This filter passes much more blue light than Congo blue, so you'll need to use three or four layers of Deep Blue, plus two or three layers of Primary Red.

The result is different than the congo blue goggles. With these goggles you can barely see anything at all, even in brightest daylight. But after about 15 seconds your eyes grow used to the dark. And then the sky looks far more black, and the plants and trees are even whiter. Humans are boring: they're all just grey-red, including clothing and hair. But human faces are weird because everyone's eyes look huge and dark.

Ditch the goggles, make an IR floodlight

In a dark room or during a moonless night these goggles are worthless. Their whole purpose is to block the background light from the environment, and if there IS no background light, then you don't need any goggles to see a bit of IR. So, if you want to experiment with direct viewing of IR LEDs or (dangerous!) IR diode lasers, just go into a well-darkened room and observe IR sources directly. But that leads to another idea: don't put filters on your *eyes*, instead put the congo blue layers over a white light source. If you have a simple theatrical floodlight that blocks any spill from the rear, and can take a colored filter in the front, then you can make a high power near-IR floodlamp. Give it a few layers of Congo Blue and one or two sheets of Primary Red to cut out the blue leakage. This is NOT the same as an 850nM LED floodlamp used with security cameras. In a dark room it looks fairly strange; appearing as dim red light until you aim it at a human face and find that their skin is translucent, their hair is wispy grey, and their eyes are alien-looking black. As usual, certain types of black cloth instead look grey (so your black Sharpie-marker artwork suddenly becomes visible.) Also, a sheet of congo blue looks nearly transparent when held in your hand. If people wear the IR goggles, the filters don't look very dark, and you can see their eyes. And psychologically its very eerie, since these effects are occuring, yet you're not wearing any goggles on your face. It might be pretty cool if used to light an "infrared art gallery" with black-on-black velvet paintings. Or if used to create incredibly intense 900nM illumination (and if this doesn't damage human eyes,) then spandex clothing worn in the gallery would appear transparent.

Speaking of art, here's an idea that requires a bright outdoor environment (such as Burning Man.) Build a booth out of transparent plastic. Cover the entire thing with layers of Congo Blue and Red. Make sure the door gives a good light-seal. Perhaps add a ventilation fan, since it'll get hot in there. Now climb inside, get used to the dark, and look around. The entire world will look like "IR goggles-view!" But that's just the first part. Now build one or two more of these booths and place them about ten feet apart. The outside observers see black shiny monolith booths, but a person inside a booths think the *other* booths are nearly transparent. Wave to the people in the other booths. Only they can see you, yet you might be surrounded by a clueless crowd outside the booths. It's almost like being invisible. Now do other things that might spring to mind. Go wild. But remember: I'M WEARING IR GOGGLES, so the "opaque" booths are transparent to me as well.

How do they work?

These IR goggles are simple: red filters block blue light, and blue filters block red... yet both colors of stagelight filters happen to pass the invisible IR light. If you stack up some blue and red filters, you get black. But it's not QUITE black, since they only block the "visible light" which has wavelength shorter than 700 nanometers. Together the two filters create an IR-pass or "lowpass" color filter.

On the other hand, human eyes are highpass filters. When you combine a lowpass filter with a highpass filter, you get a bandpass filter. When you place an IR-pass filter on human eyes, the edges of the filter responses overlap to form a pass band or sensitivity peak. The frequency of this peak is in the IR spectrum. Your eyes normally have a tiny bit of sensitivity in the IR band, but usually the bright sunlight washes it out. Wear these goggles to block out the "normal" sunlight. Your eyes have been converted into IR light sensors. Your view will be dim, but you will be seeing actual infrared light.

"Congo blue" in fig. 1 passes a hump of blue light while killing all the green, yellow, and red, but it also passes lots of IR above 700nM wavelength. "Primary red" in fig. 2 kills all the yellow, green, and blue wavelengths, but it passes IR just fine. Human eyes themselves are like a "filter" which passes green light best, but sees from violet through red, plus a tiny bit out past 700nM. Stack them all up in figure 4, and the red and blue parts get removed since the red filter absorbs blue, and the blue filter absorbs red. Now add lots more layers of congo blue, and the sloping edge of the IR band gets much sharper, so only "invisible" light from above the 710nM wavelength gets through. Use two or three layers of red filter to make sure all the blue light is suppressed. Multiply all these curves together and we get the curve in figure 5. It's a small peak, with the center frequency a little past 710nM in the infrared band. Figure 5 shows that your eyes have been converted into infrared sensors. The gain is terrible, that's why you need full sunlight in order to see any infrared scenery. Whaddaya want for under $10 bucks?!




Nothing's safe (not driving a car, not taking a shower in a bathtub!) So the sensible question is; how risky is it to use these goggles? Are they so safe that unsupervised children can use them as toys? Or should they only be used by crazy adults who enjoy hazardous entertainment?

Note that these color filters are polyester, a UV-blocking plastic, and the goggles I used included glass disks for extra UV protection. But is it enough? After wearing these goggles in bright sunlight for up to an hour at a time over many years, I've never experienced any problems such as afterimages or eye irritation. But what does the actual research say? Here is a telescope website about myths and facts regarding risks to the eyes from the sun:

Galileo, solar observing, and eye safety
The author of that website located several research studies which find that thermal eye damage occurs only if one stares at the sun for many minutes. The intensely focused light will heat a single small spot on the retina. The damage found in various studies was temporary, usually. One study from 1980 found that when staring at the sun with dilated 8mm pupils, the Infrared part of sunlight (limited to 700nM and longer) can only produce damage if one stares at the sun for 1000 seconds.

So don't use these goggles to stare at the sun for fifteen minutes.

But sunlight also includes plenty of UV wavelengths, and UV can damage the cornea in an effect called "snow blindness." Unlike thermal damage, UV damage is painless when it occurs, so you need the protection of polyester or glass sunglasses. For added protection, both Lee and Rosco sell UV-blocking filter material. If your goggles lack glass disks, it would be wise to include some layers of UV filter:


Some people argue that, if humans can see it, then it must not be infrared light. But this is wrong, or at least way oversimplified. Humans can see 750nM infrared. We can see 800, 900, even past 1000nM IR. The people objecting to this have a basic misunderstanding of bandpass filters. A bandpass will let through all frequencies: they cannot have infinitely sharp cutoffs and perfect suppression of off-band signals. Check out the human sensitivity graph below. The bandpass graph has smooth slopes at the edges of the band. For signals on these slopes, the light wavelengths farther from the passband must have higher amplitude in order to be seen. The human visual system operates as a bandpass filter with one edge usually taken to be at 700nM wavelength. If light has frequency longer than 700nM, we name it "infra-red," and it must be very bright in order to become visible. For example, colleagues report that 900nM infrared laser spots are easily visible in a dark room. (Unlike with bulbs and with IR LEDs, an infrared laser emits no shorter wavelenghts which might be seen by human eyes.)

In other words, there is no sharp division between "visible" and "infrared." We can see light which is far outside the normal passband called "visible frequencies." But that light must be intensely bright. To see 800nM infrared, just make it 1000X brighter than 700nM visible red light. To see 900nM IR, make it a million times brighter.

HVS curve to 1050nM. There's no
real border between IR and visible.

Above from "Sensitivity of the Human Eye to Infra-Red Radiation"
Griffin, Hubbard, & Wald, J. OSA V37 N7 pp546-554 (1947)

The real test of the goggles is to analyze the problem numerically. Fortunately the manufacturers of theatrical filters provide an absorption graph which can be multiplied by the Human Visual System response in order to find the sensitivity curve for eye-plus-filter. I did a crude graphical multiplication using msExcel, and I find that three layers of Congo Blue filter will shift the human eye peak response from 560nM green, pushing it out to 720nM infrared. Adding more layers pushes it deeper into the IR. Finally, when too many layers have been added, the goggles become useless. At that point, the IR peak is around 735nM, and the outdoor scene looks very strange, with light foliage beneath a black sky. Note that the HSV curve is an average over population. That curve is fuzzy, so these results are fuzzy too. Some people may see nothing through these goggles, while others may see a very bright infrared scene.


Right. Notice that I said they only work in bright sunlight? They only work in BRIGHT SUNLIGHT. How about bold italics: BRIGHT SUNLIGHT IS REQUIRED. Night vision requires either a light amplifier, or it employs an IR image-converter device, or silicon CCD video cam with an IR floodlight.

Oh, now I understand. Some eBay scammers have copied my design, only they're selling it as "night vision." They've added infrared LEDs to the sides of the goggles. No, this is just a ripoff. Don't be a victim of con artists. Other people have posted a "science project" that makes the same bogus claim. If someone pretends that some LEDs and red/blue filters can give you night vision, then that person is a hoaxer, and they're hoping you're stupid enough to fall for it. Human eyes can see infrared, but you'd need something to simulate daytime sunlight: several thousand watts of IR spotlight. But then everyone would see it, with no goggles needed. Duh, humans can see bright IR. Might as well use a visible red spotlight. See, the whole idea is a bogus ripoff.

On the other hand, it's possible to alter a cheap webcam for IR use. See many online DIY articles about removing the infrared-blocking filter from your camera. (And use congo blue on your lens to block out the visible light!) Or buy one of those $30 night vision security cams from many web suppliers. Then you'll only need an IR floodlight or filtered flashlight (cheap,) as well as a camcorder's viewfinder assembly or an LCD video display (expensive.) If you want genuine military night vision, go buy a FLIR thermal camera on eBay. They're sometimes as cheap as $1500-$2000.


No, but I think you really meant: "can these see through swimsuits?" Certain types of swimsuit are somewhat transparent out at 800-900 nanometers. I've never tried this with the goggles, so you'll have to be the first. Be sure to go to the beach wearing black goggles and an opaque Hoodie to block the leakage, then stand motionless while staring at lots and lots of swimsuits. Yeah, sure, nobody will call the cops on you. :)

On the other hand, what if you made some clothing out of congo blue filter sheets? In bright sunlight while wearing the goggles, it would appear halfway transparent. I wonder if you can find flexible vinyl Congo Blue filters, instead of the crinckely mylar type.


fig. 1

fig. 2

fig. 3

fig. 4 Crude superposition. The 700nM region is interesting

fig. 5 IR vision: human eyes wearing the goggles
(multiply Congo Blue times Human Sensitivity Curve)

fig. 6 HVS curve to 1050nM

Above from " Sensitivity of the Human Eye to Infra-Red Radiation"
Griffin, Hubbard, & Wald, J. OSA V37 N7 pp546-554 (1947)



DANGER! DON'T STARE AT THE SUN!!!! I've worn the IR goggles for hours at a time in bright sunlight. I'm not blind yet. The dark goggles make your pupils open up wide, but then the goggles don't block all that IR sunlight. Is this harmful? It hasn't hurt me yet, but I don't know if it's totally safe. At the very least, don't stare at the sun while wearing the goggles, you might get a nasty suprise later on (called Snow Blindness. Your cornea surface rots and sticks to your eyelids. Ewww.)
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